#> combinations(3)
#     [,1] [,2] [,3]
#[1,]    1    2    3
#
#> combinations(2,1:3)
#     [,1] [,2]
#[1,]    1    2
#[2,]    1    3
#[3,]    2    3
#
#> combinations(2,c("A","B","C","D"))
#     [,1] [,2]
#[1,] "A"  "B" 
#[2,] "A"  "C" 
#[3,] "A"  "D" 
#[4,] "B"  "C" 
#[5,] "B"  "D" 
#[6,] "C"  "D" 
#
combinations <- function(x, string=1:x) { 
  rbind(subsets(length(string), x, string))
}

subsets <- function(n, r, s = 1:n) { 
  if(mode(n) != "numeric" || length(n) != 1 
     || n < 1 || (n %% 1) != 0) stop("bad value of n") 
  if(mode(r) != "numeric" || length(r) != 1 
     || r < 1 || (r %% 1) != 0) stop("bad value of r") 
  if(!is.atomic(s) || length(s) < n) 
    stop("s is either non-atomic or too short") 
  fun <- function(n, r, s) 
    if(r <= 0) vector(mode(s), 0) else if(r >= n) s[1:n] else 
    rbind(cbind(s[1], Recall(n - 1, r - 1, s[-1])), 
          Recall(n - 1, r, s[-1])) 
  fun(n, r, s) 
}